Turbochargers are provided on an engine to deliver air to the engine intake at a greater density than would be possible in a normal aspirated configuration. This allows more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight.
Generally, turbochargers use the exhaust flow from the engine exhaust manifold, which enters the turbine stage of the turbocharger at a turbine housing inlet, to thereby drive a turbine wheel, which is located in the turbine housing. The turbine wheel is affixed to one end of a shaft that is rotatably supported within a bearing housing. The shaft drives a compressor impeller mounted on the other end of the shaft. As such, the turbine wheel provides rotational power to drive the compressor impeller and thereby drive the compressor of the turbocharger. This compressed air is then provided to the engine intake as described above.
There are circumstances in which it is desirable to take some of the high pressure exhaust gas flow from the engine before it reaches the turbine wheel, and redirect it to the engine air intake along with pressurized air from the compressor section of the turbocharger. For example, Exhaust Gas Recirculation (EGR) can reduce NOx emissions (e.g., NO (nitric oxide) and NO2 (nitrogen dioxide)) of the engine.
In order to direct some exhaust gas flow to the engine intake, some known exhaust system configurations include valves that block exhaust flow from the engine in order to drive up pressure and direct flow to the engine air intake. However some of these configurations lead to pressure increases across all engine cylinders that are sufficient so that the engine has to work harder to push the exhaust into the exhaust system. This in turn leads to engine pumping losses and thus engine inefficiencies.